It's in part because the US has a storied history of corrupt unions and their affiliation with the mafia and organized crime.

There's a related facet in that, to an outside observer, the UAW chased American automakers out of the country through unsustainable demands for wages and benefits.

Another part comes from the direct experience of many Americans as members of unions and some portion (we can argue percentages) of those people arrive at the conclusion that the union at best isn't worth the dues and at worse is pathological, in some cases by protecting underperformers and in others by lacking a spine when it is needed. This is where my personal experience the Teamsters and vicarious experience via my wife's membership in the NEA landed me.

It's also in part because many Americans have direct experience working alongside unions and some (again we can argue percentages) become frustrated with the rules and the pace. I've had some experience with this in the HVAC industry and in home building. I was already tainted a bit by my experience as a member above so I'm sure there was some confirmation bias here.

Lastly America has a pretty strong ethos, or myth if you prefer, of individualism and some unions and union members lay on a very thick collectivist twang in their communication that can be off-putting.

I'm not ideologically opposed to unions in any way, I just haven't seen one do a great job in the US. I hope Kickstarter is able to pull off a good example and am all for workers shooting their shot if they feel it is a good idea. I'm just not particularly optimistic.